Cleaning operations are becoming more of a burden on industry because of increasing strictness in environmental regulations relating to disposition of compounds emitted as a result of cleaning operations. The cleaning operations affected include the cleaning of clothing, rugs, and furnishings as well as industrial cleaning applications where solvent is used to degrease metals, ceramics, plastics, and other materials.
There are two types of solvent cleaning processes: open and closed. Open systems include solvent vapor degreasing, solvent ultrasonic cleaning, cold or hot solvent dipping and solvent spraying systems. Open solvent cleaning systems contaminate the environment and are relatively costly to operate due to the cost of constantly replenishing non-recoverable solvent. Also, add-on equipment to contain solvent vapor and to properly dispose of solvent vapor and liquid waste is also costly.
Closed solvent cleaning systems attempt to address the problems of open systems by maintaining an airtight seal in the cleaning chamber while solvent is being applied to the parts that are being cleaned. However, solvent is still lost in closed systems. Although the system is closed when the solvent is introduced into the cleaning chamber, the solvent still mixes with air inside the cleaning chamber. After the cleaning operation, the liquid solvent can be easily separated from the air, but the vapor solvent is more difficult to remove, even if the vapor solvent is first condensed. The vapor escapes when the cleaned parts are removed because the cleaned parts include solvent which remains on the surfaces and in the pores of the parts. Attempts to recover this solvent are expensive and less than totally successful.
Another technique for eliminating potentially contaminating solvent is incineration. However, incineration requires significant investment in special equipment, uses extra heat energy, and destroys solvent that must then be replenished for subsequent cleaning cycles. Steam stripping, another solvent elimination technique, recovers the solvent but requires special equipment and heat energy to make the steam. Also, steam stripping requires that the steam be condensed to water and then separated from the solvent, which requires additional energy input to the system.
Conventional cleaning systems have other problems in the area of hazardous emissions and solvent recovery. They are generally limited to operating at specific temperatures and pressures. They typically do not dry all the solvent off the objects before exposing them to the atmosphere. They utilize heat energy during a substantial part, if not all, of the cleaning cycle. Conventional systems also need a great deal of solvent to fill their cleaning tanks and require additional energy input to pump the solvent through the system. In addition, some solvent vapor cleaning systems must use solvents whose vapors are heavier than air. These vapors are maintained in a blanket over the boiling solvent by using expensive refrigerator coils and by limiting the dimensions of the system tank. These systems operate at fixed temperatures which are determined by the boiling point of the solvent at atmospheric pressure.
A different approach, known as vacuum degreasing, avoids some of the difficulties associated with solvent cleaning. In this approach, the contaminants are exposed to a high temperature, low-pressure environment in order to reach pressures below the vapor pressure of the contaminant. Essentially the contaminant is boiled off the parts. However, some contaminants have a very low vapor pressure and consequently require extremely high vacuums and/or temperatures for vacuum degreasing. Furthermore, although solvents are not used, the contaminants from the part may be pollutants and therefore cannot be released to the environment. Also, vacuum degreasing can be costly due to the subtorr pressures and high temperatures required. In many cases, non-volatile residue, either present in the contaminant originally (i.e., sulfur residue) or residue resulting from a breakdown of the contaminant due to the high temperature requirements (e.g. carbon deposits) are often left behind on the parts. The pollution abatement energy costs and cleaning efficiency requirements strongly limit the applications of such systems.
Furthermore, often filters are used to remove contaminants from gasses being emitted by solvent cleaning systems. The filters can use a material such as activated carbon to absorb the contaminants. However, once these filters become saturated, they must be purged of contaminants prior to further use. Methods of purging include air or steam stripping, both of which result in transferring the contaminants to the steam or the air, respectively, thus requiring the steam or the air to then also be purged of contaminants at additional cost.